How should I handle an ex-girlfriend who is now my boss?

My ex has just been made my boss, and I'm convinced I will be heading for a fairly tough time of things as I believe she will want to ert her new authority over me. I'm 31, she's 24, but has always been a real 'go getter' and strong-willed person. She told me on starting last week that I am not to address her by her first name (as others of my level may), but must address her as 'Miss Barrett' or 'Ma'am'. This is apparantly to ensure 'there is an even playing field and no allegations of favouratism'. How should I approach this - try to ride it out, or take it over her head if she is not responsive to my requests for what I would percieve to be equal treatment?